Unless someone has started reproducing them EXACTLY.
And remember that the 1969-up long branch manifolds - like those on our (two) 428HO 1969 Grand Prixs - didn't have the flapper either.
We've got an extra pair of manifolds off another 428HO GP - I'll dig them out and measure the outlets.
And, as a side-note, Lance has listed an NOS RH manifold WITH the flapper - for the extremely hard-core restoration. $2800.
https://www.ebay.com/itm/11455443956...AAAOSwTEpamuEN